Data carriers, such as value documents or identification documents, bank notes, deeds, checks, but also other objects of value, such as branded articles, are often provided for protection with security elements which permit a check of the authenticity of the data carrier and which at the same time serve as protection from unauthorized reproduction. The security elements can be configured for example in the form of a security thread embedded in a bank note, a cover foil for a bank note with a hole, an applied security strip or a self-supporting transfer element, such as a label, which is applied after its production to the value document. Another variant is for example a tear thread for product packages.
A special role is played here by security elements having optically variable elements which convey different pictorial impressions to the viewer from different viewing angles, because optically variable elements cannot be reproduced even with high-quality color copiers. The security elements can be equipped for this purpose with security features in the form of optically-diffractively active micro- or nanostructures, e.g. with conventional embossed holograms or other hologram-like diffraction structures, as described for example in the prints EP 0 330 733 A1 or EP 0 064 067 A1.
It is also known to use lens systems as security features. There is thus described for example in the print EP 0 238 043 A2 or in DE 36 09 090 A1 a security thread made of a transparent material having embossed on its surface a screen comprising a plurality of parallel cylindrical lenses. The thickness of the security thread is so chosen that it corresponds approximately to the focal length of the cylindrical lenses. A printed image is applied in exact register to the opposite surface, the printed image being designed with consideration of the optical properties of the cylindrical lenses.
For some time so-called moire magnification arrangements have also been used as authenticity features. Such a moire magnification arrangement is disclosed in the print WO 2006/087138 A1 or in DE 10 2005 028 162 A1. The security element disclosed in WO 2006/087138 A1 has at least first and second authenticity features. The first authenticity feature comprises a first arrangement with a multiplicity of focusing elements which are present in a first grid, and a second arrangement with a multiplicity of microscopic structures which are present in a second grid. The first and second arrangements are disposed relative to each other such that the microscopic structures of the second arrangement are to be seen magnified upon viewing through the focusing elements of the first arrangement.
Such a magnification effect is also referred to as moire magnification. The basic mode of functioning of moire magnification arrangements is described in the article “The moiré magnifier”, M. C. Hutley, R. Hunt, R. F. Stevens and P. Savander, Pure Appl. Opt. 3 (1994), pp. 133 to 142. Very briefly, moire magnification thus designates a phenomenon which occurs upon viewing a grid comprising identical image objects through a lenticular screen with approximately the same pitch. As with any pair of similar grids this results in a moire pattern, but in this case each moire fringe appears in the form of a magnified and rotated image of the repeat elements of the image grid.
The focusing elements of micro-optic authenticity features are embossed from embossing lacquers. The microstructures to be viewed through the focusing elements can have any desired forms. According to WO 2006/087138 A1 they are produced from a colorless embossing lacquer and coated reflectively or produced from a colored embossing lacquer. So that a complete microstructure is recognizable in each case through the focusing elements, referred to hereinafter as microlenses, microlenses and microstructures must be of approximately the same order of magnitude. Moreover, the magnification effect of the microlenses is the greater the closer the microstructure is located to the focus of the lens. Since the focal length is smaller the stronger the curvature of the lens is, it is necessary for good magnification that either the distance between lens and microstructure is large (with a small lens curvature) or a strongly curved lens is employed (with a smaller distance between lens and microstructure).
The relation between the size of the microstructure, the diameter and the focal length of the lenses causes a strong curvature of the lenses and a relatively high thickness of the micro-optic authenticity feature and thus of the security element.
On account of the necessary considerable embossing depth and curvature, the focusing elements or microlenses are difficult to produce by embossing. Moreover, in the case of strongly curved microlenses the size of the microstructures to be reproduced is limited.
Another problem consists in the fact that incorporation into value documents, e.g. bank notes, is more difficult the thicker the security element is. Upon bending or creasing of the value document, thick security elements act brittle and tend to break.
In addition, embossed layers of high thickness tend to scatter light uncontrolledly, so that through thick microlenses the microstructure motif to be viewed can be outshone and its brilliance reduced.
A further difficulty is the resolution of the motifs of the microstructure arrangement. The microstructures must be in the order of magnitude of the microlenses. If complex microstructures, such as letters, numbers, logos or even images, are to be represented, the resolution must be a few micrometers, preferably considerably therebelow, i.e. in the nanometer range. Such resolutions can often not be obtained with classical printing methods as are sometimes employed for producing the microstructure motifs in conventional micro-optic authenticity features. An alternative to printing is the production of the microstructure motifs by embossing of embossing lacquers as carried out according to WO 2006/087138 A1. However, this alternative is often satisfactory only in the case of colorless embossing lacquers. Colored embossing lacquers can be colored either by pigments or by soluble dyes. Both possibilities have disadvantages. Soluble dyes tend to bleed in connection with solvents and sometimes show considerable migration. Pigments often contain particles with a particle size that is too large for completely following the embossed structure of extremely fine microstructures. This results in microstructure motifs with pigment defects that are clearly visible when magnified.